The present invention relates to a process for the preparation of metal sulphides.
Metal sulphides find various uses in technical and scientific applications. For instance, several rare earth and other metal sulphides are of specific interest as materials or components in optical glasses for the manufacture of, e.g., optical fibers and active fiber amplifiers. In this field of application there is an increased demand of new materials useful for fiber optics with improved optical properties. Typical of such materials are, for example, the sulphides of Lanthanum (La.sub.2 S.sub.3), Praseodymium (Pr.sub.2 S.sub.3), Holmium (Ho.sub.2 S.sub.3), Gallium (Ga.sub.2 S.sub.3) and Germanium (GeS.sub.2). One of the main requirements to be met for optical fiber applications, specifically for 1.3 .mu.m fiber optic systems, is ultra high purity of the sulphides, as it is expected that an increase in purity of the sulphides could lead to a substantial increase in efficiency of the fiber optics.
Several methods for the preparation of metal sulphides are known which are based on the reaction of the metal or the metal oxide with sulphur, hydrogen sulphide or other sulphur containing reagents. For a review of methods for the preparation of rare earth metal sulphides, reference is made to Gmelin Handbook of Inorganic Chemistry, 8th Edition, Rare Earth Elements - C7, page 69-74, Springer Verlag, 1983. It is apparent that the methods reported there suffer from severe disadvantages, most of all limited applicability and ineffectiveness, and do not furnish products of high purity.
For instance, high temperature reaction of the metal with sulphur in a sealed silica tube works for a limited range of metals only, where the product sulphide is miscible with excess metal, permitting a continuous reaction of the elemental charge. The partial formation of an insoluble product sulphide, such as La.sub.2 S.sub.3 may cause the reaction to cease and leads to rupture of the ampule and explosion.
High temperature reaction of the metal with hydrogen sulphide is similarly limited by the formation of insoluble surface sulphide which inhibits full progress of the reaction.
High temperature reaction of the oxide with hydrogen sulphide and/or sulphur vapour is effective with elements such as Group Ia metals only, but may leave a high level of several percent of unreacted oxide impurity. It is ineffective in the case of many rare earth oxides.
Moreover, many of the rare earth oxides require temperatures above 1300.degree. C. in this type of reaction which is inconvenient for normal furnace systems.
Carbon disulphide vapor was reported to be a more effective sulphiding agent for several rare earth metal oxides. CS.sub.2 may be supplied into the reaction system by bubbling an inert carrier gas through a bottle of CS.sub.2 liquid and passing the gaseous mixture into the reactor tube containing the oxide and which is heated to temperatures at about 1000.degree. C. This is an unpleasant and dangerous operation, since CS.sub.2 is toxic and highly flammable.
In situ generation of CS.sub.2 from sulphur and carbon in the reaction zone was also reported. Eastman et al., J. Amer. Chem. Soc, 72, 2248 (1950), reported the preparation of cerium sulphide from the dioxide by passing a stream of hydrogen sulphide over it in a carbon furnace at elevated temperatures. An intermediate formation of CS.sub.2 is speculated.